1. Field of the Invention
The present invention relates to an electrolyte composition and a photoelectric conversion element incorporating the same.
Priority is claimed on Japanese Patent Application No. 2007-311352, filed Nov. 30, 2007, the content of which is incorporated herein by reference.
2. Background Art
In general, a dye-sensitized photovoltaic cell is structured as proposed by Michael Graetzel, Switzerland, et al., Nature, United Kingdom, 1991, vol. 353, p. 737. The dye-sensitized solar cell has received considerable attention as a new class of high-conversion efficiency and low-cost photovoltaic cell (see, for example, Japanese Patent Publication No. 2664194 and Michael Graetzel et al., Nature, United Kingdom, 1991, vol. 353, p. 737).
A dye-sensitized photovoltaic cell usually includes a working electrode and a counter electrode. The working electrode has a construction in which a photosensitized dye-loaded porous film made of oxide semiconductor fine particles (i.e., nanoparticles), such as titanium dioxide, is formed on a transparent conductive electrode substrate. The counter electrode is provided opposite the working electrode. An electrolyte containing an oxidation-reduction pair is located between the working electrode and the counter electrode. Such a dye-sensitized photovoltaic cell functions as a photoelectric conversion element in which photosensitized dye sensitizes oxide semiconductor fine particles upon absorption of incident light, such as sunlight, so as to convert light energy including visible light into electricity.
A conventional volatile electrolyte solution including, for example, acetonitrile as a solvent, volatilizes especially when left outside for a long period of time. Such volatilization of the solution may cause deterioration in cell characteristics and make it difficult to provide enough durability as a device. An amorphous photovoltaic cell, for example, requires stability for 1,000 hours or more in durability evaluation test in a wet heat environment of 85° C./85% RH and in a temperature cycle of −40° C. to 90° C. To address the problem of solvent volatilization, an attempt has been made to use a non-volatile ionic liquid with high charge-transportability as the electrolyte solution (see, for example, N. Papageorgiou et. al., Journal of the Electrochemical Society J. Electrochem. Soc.), the United States, 1996, vol. 143 (10), p. 3099).
In order to obtain a photosensitized photovoltaic cell with high energy conversion efficiency which does not decrease even in a hot environment, an electrolyte composition including an imidazolium salt, water or alcohol in an amount of more than 10 wt % to less than or equal to 50 wt % and iodine has been proposed (see, for example, Japanese Unexamined Patent Application, First Publication No. 2002-289267).
An ionic liquid, however, generally has higher viscosity than that of a volatile solvent, such as acetonitrile. Such high viscosity may disadvantageously decrease a charge transport rate in an electrolyte and thus produce low output as compared to a case in which a volatile electrolyte solution is used. Although many attempts have been made to lower the viscosity of the ionic liquid, only limited materials have both stability and low viscosity. Accordingly, existing ionic liquids have not achieved low viscosity comparable to that of volatile electrolyte solutions.
Low-viscosity ionic liquids whose viscosity is decreased through addition of water are also reported. It is considered, however, that water may have various adverse effects on components of the element. Accordingly, although such low-viscosity ionic liquids have improved initial performance, it is not necessarily easy to achieve durability.
Although another attempt of mixing other volatile solvents has been made, it is still impossible to completely prevent volatilization of the solvent component. It is particularly difficult to maintain stable durability under severe durability tests as described above.